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Save the file and reboot the system. Next time when X starts, you should be able to switch between screen resolutions. The default resolution should be the best one which is supported by your display and video card ..

The solution above has been succesfully tested and currently working on my system(bt5r2 64bit kde on Thinkpad T420 laptop).
Hope this helps.
Please confirm if it works for you too.
If not, please provide more detailed information about the graphic card.
Cheers.